The present invention relates to a semiconductor device including a gate insulating film made of a material with a high dielectric constant (which will be herein referred to as a xe2x80x9chigh-dielectric-constant materialxe2x80x9d) and also relates to a method for fabricating the device.
Recently, there has been a growing demand for high-speed-operating semiconductor devices. To meet this demand, the gate insulating film of MOSFETs has been further thinned for the purpose of increasing the drivability thereof.
However, if the gate insulating film is a thin film of SiO2 (with a relative dielectric constant ∈ of 3.9), the gate leakage current increases noticeably because a tunneling current flows therethrough.
Thus, to prevent the gate leakage current from increasing while enhancing the drivability of MOSFETs, various methods for increasing the actual thickness of the gate insulating film and obtaining a desired gate capacitance have been researched. For example, according to one of those methods, the gate insulating film is made of a high-dielectric-constant material (high-xcexaK material) such as HfO2 (hafnium dioxide with a relative dielectric constant ∈ of about 30) or ZrO2 (zirconium dioxide with a relative dielectric constant ∈ of about 25).
To deposit a gate insulating film of a high-dielectric-constant material, a reactive sputtering process is performed using a target of Hf or Zr, for example, in a mixed gas ambient containing Ar (argon) and O2 gases, for example. In this manner, a gate insulating film of a high-dielectric-constant material such as HfO2 or ZrO2 can be deposited over a semiconductor substrate.
However, if the gate insulating film of the high-dielectric-constant material is deposited over a silicon substrate by the reactive sputtering method, for example, the surface of the silicon substrate is oxidized by a plasma created from the O2 gas during the reactive sputtering process. Thus, an unwanted silicon dioxide film is formed between the silicon substrate and gate insulating film. It should be noted that the unwanted film will be herein referred to as a xe2x80x9csilicon dioxide filmxe2x80x9d but can actually be any other silicon oxide film with a non-stoichimetric composition. Consequently, the gate insulating film becomes a stack of the silicon dioxide film with a relatively low dielectric constant and the high-dielectric-constant film. As a result, the gate insulating film has its effective dielectric constant decreased as a whole.
That is to say, the known method for fabricating a semiconductor device cannot obtain the desired gate capacitance. Thus, it is difficult to enhance the drivability of MOSFETs.
FIG. 7 is a cross-sectional view showing the known method for fabricating a semiconductor device.
As shown in FIG. 7, a target 80 of Zr is placed in a chamber (not shown) and a silicon substrate 90 is loaded thereto. Then, a reactive sputtering process is performed using the target 80 with a mixed gas ambient containing Ar and O2 gases created in the chamber. During this process, the surface of the target 80 is oxidized, thereby forming a Zr oxide layer 81 thereon. At the same time, the surface of the silicon substrate 90 is also oxidized to be covered with a silicon dioxide film 91. Further, as a result of the reactive sputtering process, a Zr oxide film 92 is formed over the silicon substrate 90 with the silicon dioxide film 91 interposed therebetween. Accordingly, the resultant gate insulating film becomes a stack of the silicon dioxide film 91 and Zr oxide film 92. As a result, the gate capacitance decreases compared to a gate insulating film that has the same thickness but consists essentially of a Zr oxide film alone.
It is therefore an object of the present invention to enhance the drivability of MOSFETs by getting a gate insulating film, consisting essentially of a high-dielectric-constant material alone, formed by a sputtering process without allowing any silicon dioxide film to exist on the surface of a semiconductor substrate.
An inventive method for fabricating a semiconductor device includes the steps of: a) preparing a metal target in a chamber, at least a surface region of the target having been oxidized; b) performing a sputtering process using the metal target with an inert gas ambient created in the chamber, thereby depositing a first metal oxide film as a lower part of a gate insulating film over a semiconductor substrate; and c) performing a reactive sputtering process on the metal target with a mixed gas ambient, containing the inert gas and an oxygen gas, created in the chamber, thereby depositing a second metal oxide film as a middle or upper part of the gate insulating film over the first metal oxide film.
According to the inventive method, in the step of depositing a first metal oxide film over a semiconductor substrate, i.e., the initial stage of a process for forming a gate insulating film, no reactive sputtering process is performed but a sputtering process is performed using a metal target, at least the surface region of which has been oxidized, in an ambient containing no oxygen gas. Thus, the first metal oxide film can be deposited over the semiconductor substrate without allowing any silicon dioxide film to exist on the surface of the semiconductor substrate. Also, in the step of depositing a second metal oxide film over the first metal oxide film, i.e., after the initial stage of the process for forming the gate insulating film is over, a reactive sputtering process is performed in an ambient containing an oxygen gas with the surface of the semiconductor substrate covered with the first metal oxide film. Thus, the second metal oxide film can be deposited over the first metal oxide film without allowing any silicon dioxide film to exist on the surface of the semiconductor substrate. Accordingly, the gate insulating film can be essentially made up of the first and second metal oxide films alone. In other words, a gate insulating film consisting essentially of a high-dielectric-constant material alone can be formed. As a result, the resultant MOSFET can have its gate capacitance increased and its drivability enhanced. In addition, a gate leakage current can be minimized because the gate insulating film can be thick enough with a desired gate capacitance maintained.
In one embodiment of the present invention, the step a) may include the step of performing a provisional reactive sputtering process on the metal target to be oxidized with a mixed gas ambient, containing the inert and oxygen gases, created in the chamber, thereby oxidizing the surface region of the metal target before the semiconductor substrate is loaded into the chamber.
Then, the metal target with the oxidized surface region can be prepared easily.
In this particular embodiment, the provisional reactive sputtering process is preferably performed on another semiconductor substrate that has been loaded into the chamber before the step a) is started.
Then, no insulating metal oxide is deposited on a wafer stage (which will be used as a gas-discharge electrode during the subsequent sputtering process steps) in the chamber when the surface region of the metal target is oxidized. As a result, it is possible to avoid the inability to apply a voltage to the semiconductor substrate in the subsequent process steps.
In another embodiment, the step c) may include the step of introducing the oxygen gas into the chamber with the inert gas, used in the step b), left in the chamber and with a gas-discharge continued from the step b) to carry out the reactive sputtering process.
Then, the steps b) and c) of depositing the first and second metal oxide films can be performed continuously. As a result, the throughput of the process improves.
In an alternative embodiment, the inventive method may further include, between the steps b) and c), the step of introducing the oxygen gas into the chamber with the inert gas, used in the step b), left in the chamber and with a gas-discharge for the sputtering process suspended.
Then, the mixture ratio of the inert and oxygen gases can be fixed before the step c) of depositing the second metal oxide film is started. As a result, the oxygen concentration of the second metal oxide film is controllable more easily.
In another alternative embodiment, the inventive method may further include, between the steps b) and c), the step of exhausting the inert gas, used in step b), from the chamber and then newly introducing the inert gas along with the oxygen gas into the chamber with a gas-discharge for the sputtering process suspended.
Then, the mixture ratio of the inert and oxygen gases should be fixed before the step c) of depositing the second metal oxide film is started. As a result, the oxygen concentration of the second metal oxide film is controllable much more easily.
In yet another embodiment, the step c) may include the step of supplying the oxygen gas at a controlled flow rate into the chamber to deposit the second metal oxide film with a different oxygen concentration from that of the first metal oxide film.
Then, the structure of the gate insulating film can be optimized with the reliability and the dielectric constant of the gate insulating film both taken into account. As a result, a highly reliable, high-performance MOSFET is realized.
An inventive semiconductor device includes a gate insulating film that includes: a first metal oxide film deposited on a semiconductor substrate; and a second metal oxide film deposited on the first metal oxide film. In this device, the first and second metal oxide films are made of the same type of metal oxide and have mutually different oxygen concentrations.
In the inventive device, the structure of the gate insulating film has been optimized with the reliability and the dielectric constant of the gate insulating film both taken into account. Thus, the device is implementable as a highly reliable, high-performance MOSFET.